• Making the Future: Creating and shaping future adults.
• Raising a child forces you to become more responsible and develop several survival strategies to feed, protect, and teach your child.
• Knowledge: Understanding children and enjoying a child's company and world: imagination, curiosity, smallness, etc.
• Connecting with One's Past: The childhood phase is important to everyone of us later in life, that it's good to re-visit it occasionally through children. It's a phase
As we grow old we get further from childhood years, that we forget some of our childhood memories. We even do this intentionally sometimes, which is dangerous, when we think of ourselves in our childhood as inferior beings, and of childhood as a stage of incompleteness we feel embarrassed to remember: we were not masculine/feminine enough, we were beaten up/humiliated sometimes; we are too naive, irresponsible, cowardly; we were ignored many times as well as our questions/desires/fears; and there were many other childhood issues/secrets we don't want to remember. This is not good for giving unity to one's life and treating it as a whole, which is needed for understanding, accepting and enjoying ourselves better.
Understanding childhood helps us solve many child-rearing problems and "decoding" our own "incomplete memories" or past knowledge (that Freud unnecessarily called "subconscious") when our brains were still naturally developing. Children's memory is excellent but their inability to express themselves and think logically and systematically makes childhood memories hard to fathom and understand properly.
Restrictions on life experience: Living only for a family that one is bonded to, not to larger groups of society, narrows one's mind and limits their social experience. It indirectly increases tribalism and discrimination in society. It even compels many parents to do anything for their children (committing crimes sometimes).
Children themselves can suffer more restrictions. Raising a child becomes a burden to those merely choosing it to watch a child grow up "their way": shaping his/her character, mind and fate, ignoring/unaware of the price they pay later for mindlessly following their parental instinct and satisfying their experimental whims. No wonder many celibates seek freedom from raising children and marital bonds altogether, seeing the quagmire breeders sink into.
Attachment to a child makes some parents' life miserable. They suffer whenever a child gets sick, fails an exam ... or dies. Even if he/she safely grows up, and it's time to leave, for marriage, work, travel, etc., parents feel disappointed, or even betrayed. The harms of emotional attachment/addiction are many: wasting one's time, energy, money, dignity, creativity, privacy, freedom, safety, self-control, honesty, and fairness.